1. Field of the Invention
This invention relates to a projection lens used for a three-tube television projector, and more particularly, to a projection lens which comprises a plastic lens but is not affected by variation in temperature.
2. Description of the Prior Art
In a three-tube television projector, image planes of CRTs of B (blue), G (green) and R (red) are superimposed and projected on a screen by three lenses to obtain a color image. Therefore, the projection lens used for that purpose need not be an achromatic lens but has the requirements that a large aperture having F number of from about 1.0 to 1.3 is provided and that the lens is low in cost.
Such a lens system formed from a plastic lens has been known from Japanese Patent Application Laid-Open Nos. 124,114/80, 34,515/82, 108,818/82, and the like, the lens system having a simple structure of three groups and three lenses.
The television projector has to provide a large screen whereas the entire apparatus has to be miniaturized for popularization. It is desired that a reflecting mirror is arranged in an optical system, and an optical path is bended. However, the aforesaid lens system has no space into which such a reflecting mirror may be inserted.
A projection lens is available in which though it is composed of four lenses, a plane reflecting mirror is inserted between a lens on the screen side and an intermediate lens for use with a large aperture and a wide field angle. (For example, Japanese Patent Application Laid-Open No. 198,017/83)
However, the aforesaid projection lens composed of plastic lenses is greatly varied in refractive index of plastic by temperature, and therefore, when the back-focus varies to vary the temperature within the television projector apparatus, a focal point is not formed on the screen to deteriorate a projected image. This makes it necessary to have apparatus which can minimize a variation in temperature of the projection lens, resulting in an increase in cost.
The change in the back-focus due to the temperature of the projection lens may be minimized by forming the second lens into a glass lens, for example, as disclosed in Japanese Patent Application Nos. 118,616/83 and 125,007/83. However, for accomplishment of sufficient correction of aberration, where an inexpensive material is used for a second lens whose refractive index is about 1.5, an aspherical lens has to be used, and where a spherical lens is used, a material having a high refractive index has to be used. In either case, high costs involve.
Japanese Patent Application Laid-Open No. 155,818/83 discloses one which partly comprises a glass lens. In this disclosure, a second lens is divided into one comprising glass and the other comprising plastic, in which case the plastic lens greatly serves relative to the entire refracting power, and with respect to the variation in temperature, it will be unsatisfactory in the event the focal length of the projection lens exceeds 150 mm.
None of these known patents disclose that two reflecting mirrors may be inserted and arranged in the lens system. There is Japanese Patent Application No. 174,297/83 filed by the present applicant in which two reflecting mirrors may be inserted. This application has the construction as shown in FIG. 8, which is however not only insufficient in correction relative to the variation in temperature but the distribution of the refracting power to the second lens is great, because of which an introvert comatic aberration remains in a portion where a field angle is large as shown in FIG. 9, and a quality of image in a peripheral edge portion of the image plane is not always satisfactory.